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Sturrock, P. A.

Publications and source records attributed to Sturrock, P. A..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Energy release in solar flares

On the basis of observations of energy release in solar flares, it is proposed that different processes may operate on four different time scales; (1) the subsecond scale of subbursts that prominently figure in mm-wave records; (2) the few-seconds scale of elementary bursts featured in hard X-ray records; (3) the few-minutes scale of the impulsive phase; and (4) the longer than tens-of-minutes scale of the gradual phase. It is proposed that the magnetic field concentration into 'magnetic knots' at the photosphere is of consequence to coronal magnetic field structure, so that magnetic fields in this region may be seen as an array of elementary flux tubes. It is further proposed that the gradual phase of energy release involves a steady process of reconnection, while the impulsive phase involves a more stochastic process of reconnection due to mode interaction.

Sturrock, P. A.↗
NASA NTRS1984

Extragalactic radio sources

The standard model for extragalactic variable radio sources comprises an isotropically expanding plasmoid with frozen magnetic flux and an electron distribution which evolves adiabatically. This model leads to the following relaton between the peak luminosity L (sub nu, m) and the relevant frequency nu(sub m) which are functions of time: L(sub nu,m) is proportional to nu(sub m)(n) where N = (7n + 5)/(4n + 5). In this expression, n is the spectral index in the optically thin part of the spectrum, where L (sub nu) is proportional to nu (-n). For n in the range 0.5 to 1.5, the standard model yields N in the range 1.2 to 1.4. By contrast, analysis of observational data yields estimates of N in a small range about the mean value 0.4, in clear contradiction with the standard model.

Sturrock, P. A.↗
NASA NTRS1984

Progress in the study of homologous flares on the sun. II

Studies of groups of homologous flares in active regions in 1980 were performed using a variety of space and ground-based instruments. The properties of three of the groups are described and combined to form a possible sequence of events.

Woodgate, B. E.↗
NASA NTRS1982

The acceleration and propagation of solar flare energetic particles

Observations and theories of particle acceleration in solar flares are reviewed. The most direct signatures of particle acceleration in flares are gamma rays, X-rays and radio emissions produced by the energetic particles in the solar atmosphere and energetic particles detected in interplanetary space and in the Earth's atmosphere. The implication of these observations are discussed. Stochastic and shock acceleration as well as acceleration in direct electric fields are considered. Interplanetary particle propagation is discussed and an overview of the highlights of both current and promising future research is presented.

Forman, M. A.↗
NASA NTRS1982

Examination of time series through randomly broken windows

The study of many astronomical problems involves a consideration of time series. The present investigation was prompted by a problem related to the determination of normal modes of oscillation of the sun. It is desirable to have an approach for obtaining an estimate of the accuracy with which oscillation modes may be determined when it appears possible to observe the sun over a long interval of time losing some blocks of time because of cloud cover. The present investigation has the objective to develop a model for addressing problems of this type. After presentation of a few general formulae, the problem is considerably simplified by supposing that observations are made over a large number N of equal time intervals, each of length t, so that the total time interval T is given by T = Nt. With certain additional simplifying assumptions, attention is given to the statistical properties of two functions which are representative of 'randomly broken' window functions.

Sturrock, P. A.↗
NASA NTRS1982

The cooling and condensation of flare coronal plasma

A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The important features of this model which distinguish it from previous models of flare cooling are: (1) Most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation. (2) Flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints. (3) The differential emission measure has a strong temperature dependence. These results can account for recent observations of compact flare loops that are not consistent with the previous cooling models.

Antiochos, S. K.↗
NASA NTRS1981

Temperature minimum heating in solar flares by resistive dissipation of Alfven waves

The possibility that the strong heating produced at temperature-minimum levels during solar flares is due to resistive dissipation of Alfven waves generated by the primary energy release process in the corona is studied. It is shown how, for suitable parameters, these waves can carry their energy essentially undamped into the temperature-minimum layers and can then produce a degree of heating consistent with observations.

Emslie, A. G.↗
NASA NTRS1981

Examination of time series through randomly broken windows

In order to determine the Fourier transform of a quasi-periodic time series (linear problem), or the power spectrum of a stationary random time series (quadratic problem), data should be recorded without interruption over a long time interval. The effect of regular interruption such as the day/night cycle is well known. The effect of irregular interruption of data collection (the "breaking" of the window function) with the simplifying assumption that there is a uniform probability p that each interval of length tau, of the total interval of length T = N sub tau, yields no data, is investigated. For the linear case it is found that the noise-to-signal ratio will have a (one-sigma) value less than epsilon if N exceeds p(-1)(1-p)epsilon(-2). For the quadratic case, the same requirement is met by the less restrictive requirement that N exceed p(-1)(1-p)epsilon(-1).

Sturrock, P. A.↗
NASA NTRS1981

Coronal heating by stochastic magnetic pumping

Recent observational data cast serious doubt on the widely held view that the sun's corona is heated by traveling waves (acoustic or magnetohydrodynamic). It is here proposed that the energy responsible for heating the corona is derived from the free energy of the coronal magnetic field derived from motion of the 'feet' of magnetic field lines in the photosphere. Stochastic motion of the feet of magnetic field lines leads, on the average, to a linear increase of magnetic free energy with time. This rate of energy input is calculated for a simple model of a single thin flux tube. The model appears to agree well with observational data if the magnetic flux originates in small regions of high magnetic field strength as proposed by Tarbell et al. (1979). On combining this energy input with estimates of energy loss by radiation and of energy redistribution by thermal conduction, scaling laws are obtained for density and temperature in terms of length and coronal magnetic field strength.

Sturrock, P. A.↗
NASA NTRS1981

The cooling and condensation of flare coronal plasma

A model is investigated for the decay of flare heated coronal loops in which rapid radiative cooling at the loop base creates strong pressure gradients which, in turn, generate large (supersonic) downward flows. The coronal material cools and 'condenses' onto the flare chromosphere. The features which distinguish this model from previous models of flare cooling are: (1) most of the thermal energy of the coronal plasma may be lost by mass motion rather than by conduction or coronal radiation; (2) flare loops are not isobaric during their decay phase, and large downward velocities are present near the footpoints; (3) the differential emission measure q has a strong temperature dependence.

Antiochos, S. K.↗
NASA NTRS1980

Examination of time series through randomly broken windows

The effect of irregular interruption of data collection (the breaking of the window function) on the spectrum determination of a time series is investigated. It is assumed that there is a uniform probability p that each interval of length tau, of the total interval of length T = tau, yields no data. For the linear case (Fourier transform) it is found that the noise to signal ratio has a (one sigma) value less than epsilon if N exceeds p(-1) (1-p) epsilon (-2). For the quadratic case, the same requirement is met by the less restrictive requirement that N exceed p(-1) (1-p) epsilon (-1). It appears that, if four observatories spaced around the Earth were to operate for 25 days, each for six hours a day (N = 100), and if the probability of cloud cover at any site on any day is 20% (p = 0.8), the r.m.s. noise to signal ratio is 0.25% for frequencies displaced from a sharp strong signal by 15 micro Hz. The noise to signal ratio drops off rapidly if the frequency offset exceeds 15 micro Hz.

Sturrock, P. A.↗
NASA NTRS1980

Coronal heating by stochastic magnetic pumping

Recent observational data cast serious doubt on the widely held view that the Sun's corona is heated by traveling waves (acoustic or magnetohydrodynamic). It is proposed that the energy responsible for heating the corona is derived from the free energy of the coronal magnetic field derived from motion of the 'feet' of magnetic field lines in the photosphere. Stochastic motion of the feet of magnetic field lines leads, on the average, to a linear increase of magnetic free energy with time. This rate of energy input is calculated for a simple model of a single thin flux tube. The model appears to agree well with observational data if the magnetic flux originates in small regions of high magnetic field strength. On combining this energy input with estimates of energy loss by radiation and of energy redistribution by thermal conduction, we obtain scaling laws for density and temperature in terms of length and coronal magnetic field strength.

Sturrock, P. A.↗
NASA NTRS1980

Is the galactic corona produced by galactic flares

The effect of the differential rotation of the disk of the galaxy on magnetic field which penetrates the disk is considered. The magnetic field will be progressively distorted from a potential (current-free) form and will at some stage become unstable. We expect, from knowledge of solar flares, that an MHD instability, a resistive instability, or a combination of the two, will result in the release of the excess magnetic energy, and that part of the released energy will be converted into heat. By estimating the energy release and the rate at which this process will occur and by assuming that this energy input is balanced by radiation, we obtain estimates of the parameters of the resulting plasma. It appears that this process alone can heat a galactic corona to temperatures of order one-million K.

Sturrock, P. A.↗
NASA NTRS1980

A sunspot periodicity and the solar rotation

A least squares power spectrum analysis of daily sunspot numbers for the last 122 years yielded a statistically significant peak at 12.0715 plus or minus .002 days period. This feature at 11.685 days (sidereal) of the sunspot spectrum is discussed in relation to the peak at 12.22 days (sidereal) which Dicke found in his oblateness data. The data is attributed to the Sun's core if the core rotates at either 12.0715 days or 24.1430 days period (synodic). It is suggested that spacecraft observations combined with correlative analysis of solar surface features between eastern and western hemispheres could further reveal a basic core periodicity. A Dicke type space oblateness experiment is discussed for providing better photospheric observations than a ground instrument to determine the core periodicity.

Knight, J. W.↗
NASA NTRS1980

Flare models

The current status of solar flare modeling is reviewed. Primary and secondary observational features that a proposed flare model should be capable of explaining are discussed, including energy storage and release, particle acceleration, mass ejection, heating of the temperature minimum region, X-ray, EUV, UV, visible and radio emission and mass flow. Consideration is then given to the twisted flux tube paramagnetic recombination model of Gold and Hoyle (1960), the current model of Alfven and Carlqvist (1967), closed current-sheet models such as those of Syrovatskii (1966, 1969, 1977) and Uchida and Sakurai (1977), open-field models such as those of Carmichael (1964) and Barnes and Sturrock (1972), the emerging flux model of Heyvaerts and Priest (1974, 1977, 1978) and the loop-flare models of Spicer (1977) and Colgate (1978). It is noted that no one model can yet account for all the observational features, and that there may turn out to be several types of flare, each requiring its own explanation.

Sturrock, P. A.↗
NASA NTRS1979

Positron production by pulsars

Analytical calculations based on the 'polar-cap light-cylinder' (PCLC) (Sturrock, 1971a) and 'polar-cap force-balance' (PCFB) (Roberts and Sturrock, 1972,3) pulsar models show that in either case the intensity of the 511 keV gamma-ray line from the galactic center can be explained as the result of the annihilation of positrons produced by pulsars. The Crab pulsar (braking index approximately 2.5) is estimated to have a power budget of 10 to the 38th ergs/s, and to produce a total number of 10 to the 51.7th positrons. It is suggested that positron annihilation can be supplied by pulsars if the birthrate of pulsars in the galactic center is 10 to the -8.2nd/s.

Sturrock, P. A.↗